Generally, a vacuum cleaner draws a dust and debris containing air from a surface to be cleaned, separates at least some of the dust and debris from the drawn dust and debris containing air flow in a separating unit and releases an air flow relieved of the separated dust and debris to the surrounding air. Normally, the separated dust and debris is collected in the vacuum cleaner, for example in a compartment of the separating unit or in a separate compartment or container.
One known separating unit for a vacuum cleaner is a filter bag, which is a bag made of a suitable filter material. The filter bag is placed in the air flow through the vacuum cleaner such that the air flow is forced through the filter bag, wherein dust and debris contained in the air flow is trapped inside the filter bag and the remaining air flow continues to an outlet of the vacuum cleaner. When the filter bag is full, it normally is replaced by a new one, but there exists also filter bags which are of such quality that they can be emptied and cleaned for being reused several times.
Another known separating unit for a vacuum cleaner is a cyclone or a cyclone-like separator. A cyclone separator is mounted in the air flow of the vacuum cleaner, wherein an air stream passes through the cyclone separator. A cyclone separator normally comprises an air inlet for introducing a dust and debris containing air stream into a cyclone chamber. The dust and debris containing air stream makes a tangential entry through the air inlet into the cyclone chamber and swirls around in the cyclone chamber, whereby dust and debris are separated from the air stream due to centrifugal forces. Dust and debris are caught at the cyclone chamber wall and travel along the wall to the bottom of the cyclone chamber. Many cyclone separators are provided with a dust container into which dust and debris are thrown from the cyclone chamber bottom. The air stream, which has been relieved of dust and debris in the cyclone chamber, leaves the cyclone chamber through an air outlet and continues through the vacuum cleaner to an outlet thereof.
A problem with known cyclone or cyclone-like separators is that each specific cyclone or cyclone-like separator has a limited range of operation. For example, a cyclone separator can be designed to have a high separation efficiency, wherein the term efficiency is used in relation to the ability to separate fine particles. However, the high flow resistance of such cyclone separators limit their usability in situations when a high flow rate is needed. One such situation is normal vacuum cleaning, wherein a certain air flow through the nozzle is needed in order for the vacuum cleaner to be able to pick up dust and debris from the surface to be cleaned. On the other hand, a cyclone separator, which is designed to have a low flow resistance may have a separation efficiency that is too low for separating finer dust particles. Another example is the position of the air inlet of the cyclone, which restrains how the cyclone can be connected to an air stream through the vacuum cleaner.